Human uracil-N glycosylase is purified from normal full term placentas. The purification follows standard ion exchange chromatographic techniques followed by isoelectric focusing. Separation of the accompanying endonuclease activity is being effected through the use of chaotropic agents and selective denaturation techniques. Modified thymine derivatives in DNA arise from oxidation and/or free radical reactions and may be the substrate for the endonucleolytic activities present in both pro and eukaryotic cells directed against oxidized and irradiated DNA. We have developed a rapid purification of 5,6-dihydroxydihydrothymine (which is thought to be the principal modification of DNA after oxidation or irradiation), and will use this as a marker to determine the specificity of enzymatic activity directed against such modified DNA.